This invention relates generally to blemish detection and particularly to the detection of blemishes in the phosphor screen of a kinescope faceplate, or panel, while simultaneously avoiding the detection of deposits on the outside surface of faceplate as blemishes.
U.S. Pat. Nos. 4,454,541 and 4,454,545 describe various aspects of a system for detecting blemishes in various objects, such as the phosphor screens on kinescope faceplates. The inventions described in these patents are very satisfactory for most purposes. However, some difficulties have been encountered when they are utilized to detect blemishes in the phosphor screens on kinescope faceplates. Kinescope faceplates typically have a slightly curved viewing screen integral with substantially parallel sidewalls. The phosphor screens are formed by depositing stripes, or dots, on the concave inside surface of the faceplate. For a color kinescope, three different phosphors are applied, each of which emits a different color of light when impacted by electrons. Additionally, when viewed from the viewing or front side, the phosphor stripes, or dots, of which the screen is composed, are surrounded by a thin layer of black matrix material. When viewed from the screen side, the phosphors overlap the matrix material. The phosphors and the black matrix material are deposited by covering the entire internal surface of the faceplate with a photosensitive slurry containing the material to be deposited. An apertured shadow mask is inserted into the panel and the photosensitive slurry is exposed to light through the apertures. Because four different materials comprise the screen, a substantial amount of handling and processing is required during the formation of the screen. Additionally, the environment in which such processing takes place cannot be as clean as might be desired because of the inherent nature of the slurry application and the processing steps. For these reasons, there is a tendency for unwanted deposits, such as matrix material and phosphor splashes, or dust and dirt, to adhere to the outside surface of the faceplates.
After the screens are completely formed and a shadow mask is permanently inserted, the faceplates are permanently affixed to a funnel portion, and the final assembly steps are undertaken. A substantial number of parts are added during the final assembly and a substantial number of processing steps are required. For this reason, prior to mating a faceplate with a funnel portion, it is preferable to inspect the phosphor screens for blemishes, such as missing phosphor portions, or extra matrix material, so that screens having such blemishes are identified early in the processing stage to avoid the substantial expense of mating unusable faceplates with funnels. The inventions described in U.S Pat. Nos. 4,454,541 and 4,454,545 are used to inspect faceplates for blemishes. Although the patents do not describe the details of the illumination of the faceplates, initially when inspecting screens for blemishes, the entire phosphor screen was illuminated with light which first passed through the phosphor screen, and then the panel glass prior to reaching a light detector mounted within a camera. This method of inspection is very satisfactory in detecting blemishes in the phosphor screens of clean faceplates. However, difficulties have arisen in detecting blemishes in the screens of dirty faceplates. Unwanted deposits which accumulate on the outside surface of the faceplates can be identified as blemishes and result in the rejection of an acceptable faceplate. Cleaning of the faceplates is not desirable because of the additional expense of the otherwise unnecessary cleaning. For this reason, there is a need for a method for detecting blemishes in the phosphor screen on one surface of a faceplate, or other transparent object, in the presence of deposits on the other surface of the object, while simultaneously avoiding identifying the deposits as blemishes. The present invention fulfills this long felt need.
A further requirement for a satisfactory blemish detector is that the contrast detected for a blemish of a given size should be substantially the same irrespective of the location of the blemish in the faceplate. When the screen is viewed through the glass, it is possible for a particular blemish to be detected with a contrast that varies with the position in the screen. Thus, missing phosphor can show a reduced contrast when the blemish is close to the sidewall. The present invention gives a method of avoiding this contrast variation.